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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 06, 2007
Per Share Operating Performance: |  |  |  |  |
Net asset value, beginning of period | $ 26.59 usl_NetAssetValuePerUnit |  | $ 42.83 usl_NetAssetValuePerUnit |  | $ 50.00 usl_NetAssetValuePerUnit
Total income (loss) | $ (2.58)usl_TotalIncomeLossPerUnit |  | $ 1.38 usl_TotalIncomeLossPerUnit |  |
Total expenses | $ (0.05)usl_TotalExpensesPerUnit |  | $ (0.11)usl_TotalExpensesPerUnit |  |
Net increase (decrease) in net asset value | $ (2.63)usl_IncreaseDecreaseInNetAssetValuePerUnit |  | $ 1.27 usl_IncreaseDecreaseInNetAssetValuePerUnit |  |
Net asset value, end of period | $ 23.96 usl_NetAssetValuePerUnit |  | $ 44.10 usl_NetAssetValuePerUnit |  | $ 50.00 usl_NetAssetValuePerUnit
Total Return | (9.89%)usl_RateOfReturnOnAverageAsset |  | 2.97%usl_RateOfReturnOnAverageAsset |  |
Ratios to Average Net Assets |  |  |  |  |
Total income (loss) | (8.12%)usl_PercentageOfTotalRevenue |  | 2.80%usl_PercentageOfTotalRevenue |  |
Expenses excluding management fees | 0.21%usl_ExpensesExcludingManagementFeePercentage | [1] | 0.41%usl_ExpensesExcludingManagementFeePercentage | [1] |
Management fees | 0.60%usl_ManagementIncentiveFeePercentage | [1] | 0.60%usl_ManagementIncentiveFeePercentage | [1] |
Net income (loss) | (8.32%)usl_PercentageOfConsolidatedNetIncomeLoss |  | 2.55%usl_PercentageOfConsolidatedNetIncomeLoss |  |
[1] | Annualized

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